A Photochemical Route to 2-Substituted Benzo[b]furans

Abstract

2-Substituted benzo[b]furans were synthesized by a one-step metal-free photochemical reaction between 2-chlorophenol derivatives and terminal alkynes by tandem formation of an aryl-C and a C-O bond via an aryl cation intermediate. The mild conditions and the application to chlorophenols rather of the more expensive bromo or iodo analogues makes this procedure environmentally convenient.

Flow Metal‐Free ArC Bond Formation via Photogenerated Phenyl Cations

“…Irradiation of a 2,2,2‐trifluoroethanol (TFE) solution containing 1a (0.05 M) and mesitylene (0.5 M) in a multi‐lamp apparatus (10×15 W lamps, λ =310 nm) for 24 h caused the complete consumption of the aryl chloride and compound 8 was formed in 76% yield (Table 1, entry 1). However, in order to avoid the use of such an expensive reaction medium, a 5:1 MeCN‐H 2 O mixture was likewise investigated as a feasible alternative 28a,c. A higher yield of 8 (86%) was obtained after only 18 h irradiation, along with a small amount (5% yield) of anisole 1H (entry 2).…”

Leaving Groups in Metal‐Free Arylations: Make Your Choice!

“…[30] The resulting triplet aryl cation 3 Ar + exhibits an enhanced reactivity towards π-bond nucleophiles making accessible a wide range of synthetic targets, including, among the others, allylarenes, [31] biaryls, [32] γ-lactones, [33] arylalkynes [34] and benzo[a]furans. [35] Such approach works efficiently when electron-rich arenes are involved (e.g. haloanilines, anisoles [30] and even alkylbenzenes [36] ) and was extended with some success also to aromatics bearing moderately electron-withdrawing substituents (Scheme 8).…”

“…Metal-free arylation via triplet aryl cation. [30][31][32][33][34][35] Finally, bromo and dibromonaphthols were efficiently arylated in acetonitrile in the presence of a base (triethylamine) by nucleophiles such as pyrrole and allyltrimethylsilane via photochemical cleavage of the Ar-Br bond and subsequent formation of an electrophilic carbene intermediate. [39]…”

Aromatic Chemistry in the Excited State: Facilitating Metal‐Free Substitutions and Cross‐Couplings

“…Following an acid–base reaction, the coupling product can be delivered. By elegantly engineering the substrates, the Albini group employed their metal‐free Sonogashira procedure to synthesize benzofurans [Scheme B, equation (2)] . With ortho ‐chlorophenols and terminal alkynes as starting materials, the desired benzofurans were synthesized in good yields, driven by light.…”